Electrical communication circuit



Nov.23,1937. SBALLANHNE 2,100,180

ELECTRICAL COMMUNICATION CIRCUIT Filed Dec. 17, 1929 2 Sheets-Sheet 1Nov. 23, 1937. s. BALLANTINE 2,100,180

ELECTRICAL COMMUNICATION CIRCUIT Filed Dec. 1'7, 1929 2 Sheets-Sheet 2Carrie/- l o/faqe wadu/afed L9 ww m,

r 7 character D indicates a triode vacuumtubehav- V Patented Nov. 23,1937 I g r V I ATENT orr cs ELECTRICAL COMMUNICATION CIRCUIT StuartBallantine, Mountain Lakes, N. J., assignor, by mesne assignments, toRadio Corporation of America, New York, N. Y., a corporation of DelawareApplication December 17, 1929, Serial No. 414,797

21 Claims. (01. 25027) This invention relates to electricalcommuniappropriate form to constitute a deteetorstage cation circuitsand particularly vacuum tube deof the plate-circuit rectifier type.tector circuits. In accordance with the present invention, the' Objectsof the present invention are to provide detector stage also includes acircuit element or methods of and circuit arrangements for extende entsfor autom y adj the Operate ing the range over which the audio frequencying characteristics of the tube in accordance with output voltage 'of avacuum tube detector is a rr er t ge ct fied by the tube. As shownsubstantially linear function of the radio frein the diagram, hresistance R 8/ Series quency carrier voltage of the input signal. Otherment in both the p ate and the input circuit of for stabilizing theplate resistance for detection. by acondenser C that serves as a filterto repress More specifically, objects of .the invention are to vo tageVariations of audio or modulation freprovide methods of and circuits forautomatically quency which, if allowed to enter the grid circuit,varying the operating characteristics of a vacuu d e p fi and in mostQircumstances,

ponentof the rectified current in an output cirfeet the sensitivity ofthe detector. The usual suit of the tube which is due to carriervoltage. by-pass condenser C1 is connected from the p te These and otherobjects of the invention will to Cathode 0 I -p s di eq en y currents beapparent from the following specification around the audio frequfincytransformer- 29 w taken with the accompanying drawings, in The carriervoltage is rectified in the plate cir- 20 which; cult and produces apotential drop across the re- 1 15,3, Schematic diagram of a detectorsistance R which increases with carrier voltage.

stage embodying the inventi This direct current potential drop isapplied to the :Fig. 2 is a curve sheet showing the relation beccntrolgrid m Such po'la'nty as to Increase the 'l objects are'to providemethods of and circuits the'tube. and this resistance is preferablyshunted 10 um tube detector in accordance with that comdepending upontheir phase, would adversely tween the plate resistance of detection andthe grid bias negatively as the carrier Voltage 25 carrier voltage, a ip 7 Fig. 3 is a curve sheet showing the relation between direct currentin the plate circuit and carrier voltage; V Figs. 4 to 9, inclusive, and11. arecircuit diagrams of detector stages embodying the invention, and

Fig. 10 is a circuit diagram of a radio receiver that includes adetector stage constructed in accordance with the invention. f f a i Inthe circuit shown in Fig. 1', the reference creases. The rate at whichthe automatic bias changes with carrier voltage may be adjusted over awide range by varying the resistance It.

In my copending application Ser. No. 354,022, filed April 10, 1929, nowPatent No. 2,070,640; is- 30 sued February 16, 1937, I have describedthe action of thisautomatic bias in extending the range of carriervoltage throughout which the vacuum tube acts as a linear rectifier, andinthe curves qualitatively the effect obtained when the bias is variedas a function of the carrier voltage.

V r The automatic variation of the grid bias has mg Impedance elementsand sources of current another important stabilizing action. In theassociated therewith to complete a detector-stage Case of plifi Stages,the fi ti plate in which the tube operates as jpla'te'clrcult sistanceis constant and determines the design of rectifier. The input circuitbetween control grid the output apparatus In a paper published in G andCathode includes an appropriate impedance the "Pr0ceedin gsof theinstitute of Radio Engi- Or terminals aCTOSS which the signal energy E0is neers, v01. 17, pages Ilsa-manl 1929, I have comprising Fig. 4 ofthat application is shown impressed, and, if required, asource ofcurrent teda theory of detection at highsignal such as battery E forestablishing a fixed bias on Voltages d ha e shown that the efiectiveplate the grid. Theplate circuit of the tube includes resistance i thecase of the high voltage dea p e Current S p S r indicated y tector, avariable quantity depending upon the tery Eb, and a suitable impedance,suchas the arrier voltage.

5 primary of the audio frequency. transformer T, Th curves of Fig. 2have beenplotted from for transferring the audio frequency output to andata obtained by measurement ,of the efi'ective amplifier stage,areproducer or other appropriate plate resistance at different carriervoltages, in

load device. 7 the case of modulated signals, and for a number Thecircuiteiements of the detector stage, asso of plate voltages. Theseveral solid line curves far described, are or may be of any suitableand identified by the reference numerals I 6|], I80,

200 and 220, respectively, show the variations of effective plateresistance for detection with the variation of carrier voltage on thegrid. In each instance, the detector was a triode tube of the separateheater type known commercially as a 227 tube, the grid bias being 22.5volts and the respective plate voltages 160, 180, etc., being indicatedby the reference numerals identifying the several curves. The dottedline curve Etta shows the relation between effective plate resistancefor detection and carrier voltage when the grid bias was automaticallyvaried as a function of the carrier voltage, the plate voltage beingvolts.

An examination of this series of curves shows that the variation ofplate resistance in an ordinary plate-rectifying detector withoutautomatic bias may be considerable, particularly at relatively low platevoltages. This variation may be reduced by increasing the plate voltage,but this has the serious disadvantage of greatly increasing the platecurrent, which in turn leads to rapid deterioration and failure of thetubes.

In Fig. 3, this variation of the direct current component of the platecurrent for difierent carrier voltages is shown graphically, the solidline curve ZBUF being plotted from data obtained with a fixed grid biasand with 200 volts on the plate, and the dotted curve 238A being plottedfrom data obtained when the grid bias was automatically varied for thesame plate voltage.

This stabilizing action of the automatic ias upon the effective plateresistance for detection permits a rational design of the output load tomatch an almost steady value of plate resistance With the expectationthat the fidelity will remain constant at the various signal levels. Italso restricts the dissipation of power by the plate and permits higheroperating voltages with attendant higher outputs than would otherwise bepossible.

In the detector stage shown in Fig. 4, the operation of the triode D asa plate-circuit rectifier is similar to that of the Fig. l circuit,except that the resistance R is shunted by the condenser C in serieswith a resistance R1. The input circuit is connected to the junction ofR1 and C, and the resistance R1 supplements the filter action of thecondenser to more effectively prevent the alternating plate currentsfrom being introduced into the grid circuit.

In the detector stage of Fig. 5, the screen-grid G of the tetrode D issupplied from the plate current source Eb, and the resistance R, shuntedby condenser C, is a series element of the screengrid circuit as Well asthe input and plate circuits. Since rectification takes place in thescreen-grid circuit, the rectifications' in both the plate andscreen-grid circuits are effective in generating the direct currentautomatic bias for the control grid G.

The circuit of Fig. 6 shows a general arrangement of circuits for thetetrode detector D for individually adjusting the contributions of theeffects of rectification in the screen-grid and plate circuits. As inthe other circuit arrangements, the resistance R is a series element inthe plate circuit but a jumper and an adjustable tap 2 are provided forshunting any desired portion of the resistance. The screen-grid circuitincludes the current source Ed and, by means of the adjustable tap 3,may include a portion of the resistance R. The usual radio frequencyby-pass condensers C1 are shunted across those sections of resistance Rthat are effectively in the screen-grid and plate circuits, and thecondenser C by-passes audio frequency currents around that part of theresistance R which is included, by adjustable tap t, in the inputcircuit. It will be apparent that a detector stage of this type affordswide latitude in the adjustment of the rate at which the automatic biasvoltage is varied.

The circuit of Fig. '7 comprises what is, in eifect, a specialadjustment of the general circuit shown in Fig. 6. In this arrangement,the screen-grid circuit is completed directly to the cathode, and theonly rectification effective to vary the control grid bias takes placein the plate circuit.

A second special adjusment is shown in Fig. 8, i. e., the plate circuitis completed to the cathode independently of the resistance R and therectification in only the screen-grid circuit determines the automaticbias.

The circuit of Fig. 9 represents still another adjustment of thegeneralized tetrode stage. The rectification in both the screen-grid andthe plate circuit establishes a potential drop across the resistance R,but the adjustable tap t of the input circuit is so positioned that onlya portion of this direct current potential drop is applied as anautomatic bias in the control grid G.

It will be noted that the circuits of Figs. 6 and 9 provide asimultaneous automatic variation of the operating potentials upon boththe control and screen-grid circuits since the resistance R is common toboth the plate and screen-grid circuits, and the mutual automaticvariations of their potentials, particularly that of the screengrid, isuseful in extending the range of linear operation. This extensionis dueto the automatic lowering of the screen-grid potential as the platevoltage tends to approach it.

The above described circuits for obtaining the automatic bias areespecially convenient when the detector tube is of the indirectly-heatedtype and the operating voltages are derived from an alternating currentpower line through a rectifier-filter combination. In Fig. 10, I haveshown the wiring diagram of a complete radio receiver that includes adetector stage embodying the present invention. Since the amplifierstages are, or may be, of any desired type, they will. not be describedin detail.

The detector tube D includes a grid G, cathode K and heater coil H. Theplate and screen-grid circuits are energized from the voltage divider 5that is supplied from the rectifier it through a filter that includes aseries inductance l and shunt capacities 8. The low potential side ofthe rectifier ii and voltage divider 5 are grounded, but the cathode Kof the detector tube is not at ground potential. The resistance R,shunted by the audio frequency by-pass condenser C, is connected betweenground and the cathode, and the potential drop across this resistance,due to rectified carrier voltage, is applied to the grid G as anautomatic bias.

When it is desired to connect the cathode to a terminal of the inputcircuit, the radio frequency by-pass condenser C may be connected asshown in Fig. 11 between the grid and the other terminal of the inputcircuit, the direct current circuit including the fixed bias source E0and the automatic bias resistance R being connected between the grid andcondenser C Normally there is no grid detection, but if the appliedsignal voltages are such as to cause a positive grid current to flowduring a part of the cycle there may be some rectification in the grid.If C is chosen sufficiently large, however, there will be no griddetection but there willzbe a changein the average grid voltage due'tothe flow of rectified grid:

current. through R. This method'of automatically varying the grid biasis described and claimed in my copending application, andlit may beemployed to assist the efiects of the platerectification of carriervoltage. The constants of. the circuit of Fig. 11 may therefore be sochosen that theautomatic control of the operat ing characteristics ofthe tube is obtained by combination of the methods described inthis andin' my earlier application.

It will be apparent. that there is considerable latitude in the designand construction of detector stages, and that the methods hereindescribed may be carried: outwith arrangements differing from thoseshown in the accompanyingdrawings. 7

I claim:

1. The method of operating a vacuum tube'detector of the screen gridtypewhich includes the step of adjusting the polarizing potentials toefiect linear detection for a limited range of input voltages,amplifying received signalsto bring the'detector input. voltages to a.value not less than those falling within said limited range, andextending the range ofinput voltages for which linear detection obtainsby applying to oneelement of the tube a. bias voltage derived from therectified current in; both the screen grid and anode circuits ofthe'detector.

2. The method of operating a vacuum tubedetector of the screen grid typewhich includes the step of adjusting the polarizing potentials to effectlinear detection for a limited range of input voltages, amplifyingreceived signals to bring the detector input voltages to a value notless than those falling within said limited range, and extending therange of input voltages for which linear detection obtains by applyingto one circuit of the tube a bias voltage derived from the potentialdrop established by rectified signal voltage in'boththe screen grid andanode circuits of the detector.

3. The method of operating a vacuum tube de- 7 tector. of the screengrid type which includes the step of adjusting the polarizing potentialsto effect linear detection for a limited range of input voltages,amplifying received signals to bring the detector input voltages to aValue not less than those falling within said limited range, and

' extending the range of input voltages for which linear detectionobtains by applying to-the conadjusting the control grid bias inaccordance with the rectified signal voltage developed in both thescreen grid and anode circuits of the detector.

5.'In a receiver for carrier wave signals, the

' combination with a vacuum tube having signal grid, screen-grid andanode electrodes, a plurality of electrode circuits, sources of currentand impedance elements cooperating therewith to complete the circuitsofa transrectifier, and means for amplifying received carrier waveenergy to bring the rectifier input voltage to a value not substantiallyless than that correspondingto linear. responseof the detector. tube,.of means in said screen grid. and anode electrode circuits'forcontrolling. the direct. current operating voltage of the signal gridelectrode circuit in accordance with rectified signal voltage in saidfirst electrode circuits.

6. In a receiver for carrier wave signals, the combination with a vacuumtube having signal grid, screen grid and anode electrodes, a pluralityof electrode circuits, sources of current and impedance elementscooperating therewith to complete the circuits of a'transrectifier, andmeans for amplifying received carrier wave energy to bring. therectifier input voltage to a value not substantially less than thatcorresponding to linear response of the detector tube, of an impedancein saidscreen grid and anode electrode circuits across which rectifiedsignal voltage establishes a potential drop, the signal gridelectrodecircuit of said tube being so associated with said impedancethat the said potential drop automatically controls the direct currentoperating potentialapplied to said signal grid electrode circuit.

7. Ina detector stage, the combination with a vacuum tube of the screengrid type having input, screen grid. and output circuits, of means forextending the range of detector input voltages for which said tubefunctions as a linear rectifier, said'means comprising a resistancecommon to all said mentioned circuits.

8. The combination of an electric discharge device of the screen gridtype provided with asignal gridfor controlling the current transmittedbetween'its cathode and anode and that between its cathode and screengrid, means providing screen grid and. anode potentials and means forsubjecting said signal grid toa bias potential which. tends to reducesaid currents substantially to zero, said potentials being of suchvalues that said device operates as a bias detector for substantiallymaximum rectification, means for causing said currents to flow, andmeans for applying tosaid signal grid a bias potential controlled bysaid currents.

9i In the art of rectifying modulated radiofrequency energy by athermionic detector tube having biasing means common to the input andoutput circuits thereof, the method which comprises, impressingmodulated radio-frequency signal-energy'upon the input circuit 'of saidtube, deriving'from said radio-frequency energyin the input circuit ofthe tube by a combination of resistance and capacity solely in the inputcircuit of. the tube a steady negative grid biasing potential,rectifying the signal energy in the anode circuit of said tube toproduce variation of the anode current at modulation frequencies,and-substantially eliminating by said combination of resistance andcapacity variation of the grid potential at modulation frequencies bysaid variations of anode current in said biasing means.

10. Radio receiving apparatus comprising a detector tube having an inputcircuit, means for im'- pressing modulated radio-frequency energy uponsaid input circuit, means for deriving a substantially steady gridbiasing potential from said impressed energy comprising an impedancenetwork solely'insaid input circuit, impedance common to the input andoutput circuits of said tube, and

means for preventing audio-frequency-variations impressing modulatedradio-frequency energy upon said input circuit, a conductive impedancetraversed by the anode current of said tube, a conductive impedance ofhigh magnitude con- 5 nected between the grid and cathode of said tubeexternal to the path of anode current and in series with said impedanceand traversed by said impressed radio-frequency energy in the inputcircuit of said tube, and a condenser of low-impedance to currents ofradio-frequency in shunt to said series-connected impedances, saidcondenser and said second impedance comprising a network for producing agrid biasing potential of magnitude to prevent degeneration byprecluding variation of the grid potential at the modulation frequenciesof said radio-frequency energy.

12. Radio receiving apparatus comprising a de tector tube, a closed loopcomprising series-connected conductive impedances and a condenser of lowimpedance to radio-frequency currents in shunt to said impedances inseries, connections from the common terminals of said impedances to theanode of said tube, and connections from the other terminals of saidimpedances to the grid and cathode of said tube, one of said impedancesderiving a grid biasing potential from the anode current of said tube,and the other of said impedances and said condenser deriving a steadygrid biasing potential from the input signal energy and preventingvariation of the first biasing potential at the modulation frequenciesof the rectified signal energy.

13. Radio receiving apparatus comprising a detector tube having an inputcircuit, means for im- 5 pressing modulated radio-frequency energy uponsaid input circuit, a conductive impedance in the anode circuit of saidtube traversed by the direct and audio-frequency components of the anodecurrent, a conductive impedance connected between the grid of said tubeand the low potential terminal of said first impedance external to thepath of anode current, and a condenser connected between the grid ofsaid tube and the high potential terminal of said first impedance, saidcondenser and said first impedance forming a network whose time constantis greater than the lowest reproduced audio-frequency of anode currentfor steady grid-biasing purposes, and which prevents degeneration byprecluding variation of the grid potential by the audio-frequency.variations --of anode current traversing said second impedance.

14. Radio receiving apparatus comprising a power detector tube having aninput circuit, means for impressing modulated radio-frequency upon saidinput circuit, a source of anode current, a resistance connected betweenthe oathode of said tube and the negative terminal of said source ofanode current, a resistance connected between the grid of said tube andsaid negative terminal external to the path of anode current, and acondenser connected between the grid and cathode of said tube, saidcondenser and said second resistance deriving a steady grid biasingpotential from the signal energy and preventing the audio-frequencyvariation of anode current through said first resistance due to platecircuit rectification of said modulated radio-frequency energy fromeffecting corresponding audio-frequency variation of the grid potentialthereby substantially preventing degenerative eiiects.

15. Radio receiving apparatus comprising a detector tube, a resistanceconnected between the cathode and anode of, said tube, and an inputcircuit comprising an inductance having one ter-' condenser and secondresistance comprising a;

network for deriving a steady grid-biasing potential from theradio-frequency signal energy in said input circuit and preventingaudio-frequency variations of the anode current traversing said firstresistance from effecting corresponding variations of the gridpotential.

16. Radio receiving apparatus comprising a detector tube, a resistanceconnected between the cathode and anode of said tube, and an inputcircuit tunable to the frequency of modulated radio-frequency energy byvariable capacity and comprising an inductance having one terminalconnected to the grid of said tube and whose other terminal is connectedto the cathode and anode terminals of said resistance by a condenser anda second resistance respectively, said condenser and second resistancecomprising a network for deriving a steady grid-biasing potential fromthe radio-frequency signal energy in said input circuit and preventingaudio-frequency variations of the anode current traversing said firstresistance from effecting correspondvariations of the grid potential.

17. A receiver comprising a detector tube, a resistance connectedbetween the cathode and anode of said tube, and an input circuit tuned.to a radio frequency and comprising an inductance having one terminalconnected to the grid of said tube and Whose other terminal is connectedto the cathode and anode terminals of said resistance by a condenser anda second resistance respectively, said condenser and second resistancecomprising a network for deriving a. steady grid-biasing potential fromthe radio-frequency signal energy in said input circuit and preventingaudio-frequency variations of the anode current traversing said firstresistance from efiecting corresponding variations of the gridpotential.

18. In radio receiving apparatus, a detector tube having anode, cathode,and grid electrodes,

a source of direct-current anode potential, a

resistance connected between the negative terminal of said source andsaid cathode and traversed by the audio frequ ncy and direct currentcomponents of the detector anode current, an input element having aterminal connected to the grid of said tube, a condenser of lowimpedance to radio-frequency currents connecting another terminal ofsaid element to the cathode of said tube, and a second resistanceconnecting said other terminal of said element to a point of said firstresistance removed from cathode to eiiect negative biasing of the grid,said condenser and second resistance comprising a network for modifyingthe grid biasing potential and whose time constant is greater than theperiod of the lowest frequency of said audio frequ ncy component of theanode current.

19. In radio receiving apparatus, a detector tube having anode, cathode,and grid electrodes, a source of direct current anode current, aresistance connected between the negative terminal of said source andsaid cathode and traversed by the audio frequency and direct currentcomponents of the detector anode current, a path in shunt to saidresistance including a condenser and a second resistance in series, saidcondenser and second resistance comprising a network for varying thegrid biasing potential in accordance with signal amplitude and whosetime constant is greater than the period of the lowest frequency of saidaudio frequency component of the anode current, means for impressingmodulated radiofrequency energy upon the input-circuit comprising aninput element connected from the grid to a point in said shunt pathbetween said condenser and said second resistance.

20. The method of preventing slumping of the radio-input audio-outputcharacteristic of a detector tube operating to effect anode circuitrectification, which comprises producing by the anode current a negativebiasing voltage applied to the grid of said tube, producing in the gridcircuit of said tube, independently of the anode current, auni-directional current derived from impressed radio-frequency signalenergy, and producing by said uni-directional current a voltage ofsubstantial magnitude, solely in the grid circuit, and supplementingsaid biasing voltage produced by anode current to increase the negativebias of said grid.

21. Radio receiving apparatus having a detector tube, means for biasingthe grid of said tube to effect rectified anode circuit currentcomprising a resistance included in the grid circuit of said tube andtraversed by anode current, a second resistance traversed solely bycurrent in the grid circuit, and a condenser in a path in shunt to apath including said second resistance, said condenser and said secondresistance forming a network whose time constant is greater than theperiod of the lowest frequency of said rectified current and offeringlow impedance to the radio-frequency input current.

STUART BALLANTINE.

